The present application claims the benefit of U.S. Provisional Application No. 60/229,322 filed Sep. 1, 2000.
The present invention relates to macroscopic patterns applied to electrodes of solid state ionic devices.
Solid oxide fuel cells (“SOFC's”) are high temperature electrochemical devices fabricated primarily from ceramic oxides. Typically they contain an oxygen ion conducting solid electrolyte, such as stabilized zirconia. The electrolyte is usually a thin dense film that separates two porous electrodes, an anode and a cathode. An interconnection is usually employed which is stable in both oxidizing and reducing environments, and provides a manifold to conduct fuel and an oxidant, usually air, separately into the cell. The cell operates by electrochemically oxidizing a gaseous fuel, such as hydrogen, to produce electricity and heat. The electrode must be compatible with the chemical, electrical, and mechanical properties such as thermal expansion rates of the solid electrolyte to which it is attached.
The use of cermet electrodes for SOFC's is well known in the art. The cermet electrode is manufactured by applying a mixture of a metallic element, an oxide, or simply yttria stabilized zirconia onto the electrolyte of a cell. Various methods are known to apply the green state cermet electrode on a solid electrolyte. Examples of such prior art methods include dipping, spraying, screen printing, and vapour deposition. In order to maximize the electrochemical active area, an electrode is applied to the entire electrolyte surface. Finally, a sintering process is usually applied to bond the cermet electrode to the electrolyte. The microstructure of a sintered cermet electrode is more amenable to modification and control, allowing the performance of the cell to be optimized.
Despite the advantages of a cermet electrode described in the prior art, the bond between a cermet electrode and the electrolyte is usually a weak one. This arises from the difference in the coefficient of thermal expansion between the cermet electrode and the electrolyte. Also the bonding between a metallic element and an oxide electrolyte relies on weak physical bonding rather than strong chemical bonding. Thus the detachment of a cermet electrode from the electrolyte is a common problem, which occurs both during SOFC manufacturing and testing. This reduces the active area for the electrode reaction, and increases the overpotential lost at the interface. This problem increases in severity as the size of the SOFC increases.
Thermal cycling capability is very important for a number of commercial applications of SOFC's. However, thermal cycling magnifies the stresses between the electrode and the electrolyte because of the difference in thermal expansion coefficients and rates. In order to suppress the problem, one solution is to increase the oxide component to enhance the bonding as well as to match the thermal expansion coefficient to an allowable value. However, this improvement is based upon sacrificing the electrical conductivity of the cermet electrode because of the reduction of the metallic component. According to the percolation theory, when the volume of the electronic conducting phase decreases toward 30 percent, the conductivity will quickly decrease. As a result, the power density of the SOFC will decrease due to the increasing electrical resistance inside the cermet electrode. To a certain extent the problems with the prior art as described herein apply to other types of electrodes in addition to cermet electrodes, such as for example, metal oxides and LSM electrodes, since there is usually a difference in thermal expansion coefficients.
Thus the optimization of the cermet electrode through composition adjustments is limited. Prior art attempts to solve the thermal expansion problems have used skeletal embedded growth of primarily ionically conducting yttria stabilized zirconia. The skeletal growth extends from the electrolyte/electrode interface into a porous metallic layer, with the composite structure comprising the porous cermet electrode. In one example, bonding of the porous nickel anode to the solid oxide electrolyte was accomplished with a modified electrochemical vapour deposition (EVD) process. This process provides well bonded anodes having good mechanical strength and thermal expansion characteristics, however overall cell performance is lower than with other bonding methods. The EVD process, while producing acceptable quality electrodes, is labour intensive and very expensive. A simpler and less expensive method of producing electrodes which mitigate the difficulties of the prior art is needed, without sacrificing electrode performance.